Geochemical exploration



United States. Patent O GEOCHEMICAL EXPLORATION Keith A. Kvenvolden,Dallas, Tex., assignor to Mobil Oil Corporation, a corporation of NewYork No Drawing. Filed Dec. 1, 1965, Ser. No. 510,978 Int. Cl. G01n31/06, 31/08 U.S. Cl. 23-230 14 Claims ABSTRACT OF THE DISCLOSURE Ageochemical exploration technique wherein petroleum source rocks arecharacterized on the basis of an acid distribution ratio, i.e., theratio of relative concentrations of even carbon number fatty acids toodd carbon number fatty acids found in rock samples. Preferably, theacid distribution ratio is determined with respect This inventionrelates to geochemical exploration for petroleum minerals, and moreparticularly to a geochemical exploration method which involves theidentification of petroleum source rock formations.

Petroleum is found in commercial quantities in subsurface rockformations such as sandstones and limestones. The presence of apetroleum deposit in a subterranean formation is not ordinarilymanifested by readily discernible indicia at the earths surface.Accordingly, various techniques have been evolved in exploring for oil.Among these are those which fall within the general classification ofgeochemical exploration.

In most geochemical exploration techniques, a search is made, usually ator near the surface of the earth, for components of petroleum,precursors of petroleum, or derivatives thereof. These procedures arebased upon the theory that these materials may have migrated to or nearthe surface of the earth from an underlying petroleum reservoir. Thus,where materials such as hydrocarbons which are normally constituentcomponents of petroleum are found, the presence of a subterraneanreservoir of petroleum in the area is indicated.

While these techniques are extremely valuable in determining thepresence of petroleum within a general area,

they usually give little indication of the location of the underlyingpetroleum reservoir. Other exploration procedures such as seismicsurveying may be utilized to identify possible petroleum reservoirs atsubterranean locations in the earths crust. However, while such measuresmay give valuable information as to subterranean structures favorable tothe accumulation of petroleum, they leave much to be desired in regardto determining which, if any, of such structures actually containpetroleum deposits.

In attempting to ascertain whether such structures contain petroleum,geologists have in recent years turned to exploration techniques basedupon the so-called source rock concept. Under this concept, it isassumed that petroleum hydrocarbons are formed during long periods ofburial in fine grained sediments of high organic matter content, thepetroleum hydrocarbons being derived from organic matter of biologicorigin which was deposited with these sedimentsin a marine environment.The exact mechanism by which oil is formed within such sediments, i.e.,source rocks, is not known with certainty. After the oil-formingmechanism has taken place, it is thought that the petroleum hydrocarbonsin the source rock then vol. 22, pp. 2-15.

3,480,396 Patented Nov. 25, 1969 "ice migrated to more permeablereservoir rocks where they accumulated in the concentrated depositsfound today.

One important series of compounds found in petroleum 0118 are the normalparaffins. Most crude oils contain approximately equal concentrations ofodd and even carbon number parafiins with the ratio of odd to evenusually falling within the range of about 0.9 to 1.15. On the otherhand, most sediments of relatively recent origin exhibit a relativelyhigh ratio of odd carbon number paraflin concentrations to even carbonnumber paraflin concentrations.

While, as noted above, the nature of the oil-forming mechanism withinsource rocks is open to question, it has been postulated that many ofthe normal parafiins found in petroleum oils are derived from relativelyhigh molecular weight fatty acids contained in the biologic materialoriginally deposited in the source rock sediment.

In current biological systems, fatty acids having an even number ofcarbon atoms vastly predominate over those having an odd number ofcarbon atoms. With the assumption that this same general relationshipexisted for the biologic material precursive of petroleum, it has beenpostulated that in the formation of petroleum the fatty acids undergo atransformation reaction in which are formed alkyl radicals that in turnreact to form fatty acids and normal parafiins having one less carbonatom than their fatty acid precursors. That is, fatty acids having Ncarbon atoms would produce fatty acids and normal paraflins each havingN-l carbon atoms. This reaction progresses until most of the fatty acidsoriginally present in the deposited biologic material have been reducedto fatty acids and normal paraflins of lower molecular weights with thedistribution between even and odd carbon number parafiins being aboutequal. For a detailed description of a theory of petroleum formation inwhich the transformation process is postulated to involvedecarboxylation of fatty acids, reference is made to Cooper, J. E. etal., A Postulated Role of Fatty Acids in Petroleum Formation, Geochimicaet Cosmochimica Acta, 1963, vol. 27, pp. 1113-1127.

In accordance with the above concepts of petroleum formation andaccumulation, it has been proposed to locate petroleum source rocks onthe basis of their normal parafiin content in exploring for petroleumreservoirs. More particularly, it has been proposed to identifypetroleum source rocks by the distribution of normal paraffins therein,with a rock having a ratio of odd to even carbon number parafiinconcentrations near 1.0, i.e., on the order of the ratio found in mostpetroleum crude oils, being indicated as a source rock. A detaileddescription of this exploration technique and certain investigationsrelative thereto may be found in Bray, E. E. et al., Distribution ofN-Paraffins as a Clue to Recognition of Source Beds, Geochimica etCosmochimica Acta, 1961,

While this technique has not as yet had extensive practical application,it is possible that experience may prove it to be a valuable tool inexploring for oil. However, several difficulties are involved inattempts to identify source rocks on the basis of their normal paraffincontent. One potential source of error stems from the possibility thatthe source rock has undergone an efficien! loss of normal paraffins tothe reservoir rock during the oil accumulation process. Thus, though thesediment is in fact a source rock, the concentration of normalparafiinsremaining therein may be exceedingly low and their distributiondifficult, if not impossible, to determine by known analytical methods.Also, in some instances, hypothecated source rocks do not occur asoutcroppings, thus necessitating the drilling of a well intosubterranean location of the source rock in order to obtain samples.Common drilling procedures often involve the use of oil as a componentin the drilling mud. Normal parafiins contained in such oil maycontaminate the samples, thus rendering an accurate analysis of thesamples impossible. Also, oil naturally produced in the Well, e.g., dueto the wellin the course of drilling penetrating an unknown reservoirrock containing petroleum hydrocarbons, is a possible source of samplecontamination.

, formula:

c n coon As noted above, a petroleum source rock may have undergone anefiicient loss of normal paraffins to a reservoir rock during the oilaccumulation process, thusrendering it diificult, if not impossible, tocharacterize the rock sediment as a source rock on the basis of itsnormal parafiin distributions. However, the migration of fatty acidsfrom the source rock to the reservoir rock during the oil accumulationperiod may have been less efiicient than that of the normal paraflins.Thus, a source rock having very low normal paraffin concentrations maycontain fatty acids whose presence and distribution may be determinedanalytically. Also, although the source rock may contain normalparaflins in discernible concentrations, the paraffin content of therock is subject to contamination as in drilling operations as notedabove. Oils added to drilling fluids or naturally produced in anexploratory well may contain fatty acids, but the concentration usuallywill be so low that any possible contamination of the rock would beslight and below the measurable range.

While the possibility of contamination of source rocks with regard totheir fatty acid content is less than the possibility of contaminationwith regard to their normal paraffin content, such contamination isconceivable and even probable under some circumstances. A possiblesource of fatty acid contamination is by deposition of modern biologicmaterial. The most common fatty acids found in natural materials have12, 14, 16, and 18 carbon atoms with those of 16 carbon atoms being themost prominent. Therefore, in accordance with one aspect of theinvention, those acids having 16 or less carbon atoms, and preferablythose acids having 18 or less carbon atoms, are excluded in thecharacterization of source rocks on the basis of their distribution ofeven and odd carbon number fatty acids.

For a better understanding of the present invention and the objectsachieved thereby, reference may be had to the following detaileddescription.

In carrying out the present invention, rock samples taken from theearths crust are analyzed with regard to their fatty acid content todetermine for each sample the ratio between the sum of relativeconcentrations of even carbon number fatty acids and the sum of relativeconcentrations of odd carbon number fatty acids. The ratios for thesamples then are correlated with each other and the locations in theearths crust from which the samples were taken to determine, inaccordance with certain criteria as explained hereinafter, possiblesource rocks of petroleum hydrocarbons. Thereafter, further exploratoryprocedues may be undertaken as described below to locate reservoir rocksto which petroleum hydrocarbons have migrated from the designated sourcerocks.

Since, as noted previously, even carbon number fatty acids are presentin relatively ample amounts in natural biologic products of either plantor animal origin while fatty acids having an odd number of carbon atomsare present only in trace amounts, it has been assumed that in asediment when it was originally deposited the ratio of even to oddcarbon number fatty acids was relatively high. This assumption isconsistent with observations that have shown even carbon number fattyacids to be much more abundant in modern sediments than odd carbonnumber fatty acids.

It also has been observed that in certain ancient sedimentary rocks theratio of even to odd carbon number fatty acid concentrations is lessthan the ratio generally found in relatively modern sediments. It is mybelief that this decreased ratio of even to odd carbon number fatty acidconcentrations, hereinafter referred to as the acid distribution ratio,in old sedimentary rocks reflects the increasing degree of degradationof fatty acids as they undergo the transformation process referred toabove in the formation of normal paraflins. It is therefore pro posedthat the ratio of relative concentrations of even and odd carbon numberfatty acids found in a particular rock is an indication of thelikelihood of this rock being a source rock; and that of a plurality ofrock formations within a geographical area and/or a geological time zonethe formation having the lowest even to odd carbon number fatty acidratio is the most likely to be a petroleum source rock. Moreparticularly, it is my hypothesis that the formation of the normalparaffins of relatively high molecular weight commonly found in crudeoils is accompanied by a degradation of the normal fatty acids in theprecursory materials to such an extent that the ratio of even to oddcarbon number fatty acid concentrations is not greater than 1.3 and inmost cases not greater than 1.2.

As a first step in practicing the instant invention, one or more samplesfrom the rock or rocks under investigation are obtained and analyzed fortheir relative fatty acid concentrations in order to determine the ratiobetween even carbon number fatty acids and odd carbon number fatty acidsin the rock or rocks. An analysis of such samples for their fatty acidcontent may be carried out by any suitable procedure. However, care mustbe taken to employ a procedure which will yield accurate results sincethe fatty acid concentration in such rocks normally will be relativelylow.

In a preferred procedure for analyzing for fatty acids the rock sampleis dried and crushed to obtain a homogeneous mixture. The acids then areextracted in the form of their salts from the rock. This may beaccomplished by adding a suitable amount of pulverized rock sample,e.g., about 500 grams, to an amount within the range of about 500 to1300 milliliters of a solution of about 10 percent by weight potassiumhydroxide and percent by weight methanol. During the addition of thepulverized rock sample, the mixture is constantly stirred. The samplethen is refluxed at a temperature of about 60 C. with stirring for abouteight hours, after which the extracted solution is recovered bycentrifuging from the mixture. The resulting solution is thereafterconcentrated by distillation. For example, the solution can beconcentrated to a volume of about 400 milliliters by distillation in atwo-foot Vigreux column.

The concentrated extract then is acidified in order to form the fattyacids. This may be accomplished by adding to about 200 milliliters ofthe extract about 500 milliliters of distilled water and sufficienthydrochloric acid to make the resulting solution have a pH of about oneor less. The concentrate then is extracted with carbon tetrachloride toremove and recover the fatty acids. This may be done by adding themixture to a separatory funnel, then extracting it successively with 50milliliters, 30 milliliters, and 20 milliliters of carbon tetrachloride.This extract then is concentrated by distillation to a volume of about10 to 15 milliliters. The concentrate then is transferred to a suitablevial and the remaining solvent is removed by employing a stream offiltered air at a temperature of about 40 C.

Following removal of the solvent, the acids are treated to form themethyl esters thereof. To the residue following removal of the solventare added 0.5 milliliter of benzene and 3.5 milliliters of a solution ofabout 10 percent by weight boron trifluoride and 90 percent by weightmethanol. This mixture then is heated in a steam bath for a period ofabout five minutes and then cooled. Thereafter, one milliliter ofbenzene and two milliliters of distilled water are added to the solutionand the resulting mixture is shaken and centrifuged. The benzene layercontaining the methyl esters then is recovered.

Following the esterification procedure, a urea adduct of the methylesters is prepared. This is accomplished by adding three milliliters ofmethanol saturated with urea to the benzene solution. The mixture isagitated and thereafter cooled to a temperature of about C. andmaintained at this temperature for a period of about 18 hours. The ureaadduct of the methyl esters appears in the form of crystals. The ureaadduct crystals are separated from the liquid by centrifuging and thecrystals thereafter are washed with benzene. This may be accomplished bywashing the crystals twice, utilizing one milliliter portion of benzeneeach time. The resulting urea adduct then is decomposed in about twomilliliters of water. The resulting solution is shaken and centrifugedand the benzene layer recovered.

The benzene layer thus recovered normally will contain, in addition tothe methyl esters of the fatty acids, normal paraffins which wereoriginally present in the rock sample. Such normal paraflins may beremoved by silica gel chromatography. In this phase of the analysistechnique, the benzene solvent in the recovered solution is removed byevaporation at 40 C. under a filtered air stream. The remaining extractis dissolved in a suitable amount, e.g., five milliliters, of normalheptane and the resulting solution is added to a nine-milliliterdiameter glass chromatographic column packed with suitable silica gel.For example, the column may be packed with nine grams of 100 to 200 meshactivated silica gel and prewet with five milliliters of normal heptane.The column then is eluted successively with three five-milliliterportions of normal heptane, one five-milliliter portion of benzene, andthree five-milliliter portionsof methanol. The methyl esters of thefatty acids are recovered in the benzene-methanol fraction collectedfrom the chromatographic column.

In many casesit will be desirable at this stage of the analysisprocedure to confirm the presence of these methyl esters in thebenzene-methanol fraction. This may be accomplished by infraredspectrometry. In this case, the benzene and methanol solvents may beremoved by evaporation at 40 C. under air. The remaining extract isdissolved in a suitable amount, e.g., about three milliliters of carbondisulfide. The resulting carbon disulfide solution then may be analyzedin an infrared spectrophotometer suitable for scanning the twotofifteen-micron region in order-to verify the presence of methyl estersof the fatty acids in the benzene-methanol fraction.

Following the infrared spectrometry, the carbon disulfide solvent isremoved by evaporation at 40 C. under air and the remaining extract isdissolved in benzene. The resulting benzene solution of methyl estersthen is analyzed by gas chromatography in order to determine therelative concentrations of methyl esters of fatty acids containedtherein. Any suitable chromatographic analysis apparatus may be employedfor this purpose. In experimental work an F&M Model 500 gaschromatograph having a 24-inch by A-inch column packed with about fourpercent silicone gum rubber on 60 to 80 mesh silanized diatomiticaggregates available under the trade name Chromosorb W has proved to besatisfactory.

In carrying out the analysis, helium may be utilized as the carrier gasand the column temperature may be increased from an initial temperatureof about 100 C. to a maximum of about 350 C. at the rate of about 5.6

'0. per minute. Thereafter, the temperature may be held at 350 C. untilthe esters are eluted completely.

The chromatographic column may be calibrated with a standard solution ofmethyl esters of fatty acids in order to determine the elution times ofthe esters of the various acids under investigation. In experimentalprocedures involving this analysis technique, a gas chromatograph of theabove type was calibrated with a standard solution containing methylesters of normal fatty acids having 18 and 26 carbon atoms per molecule.Retention times and instrument sensitivities for other methyl esters ofnormal fatty acids were obtained by interpolation and extrapolation ofthe calibration curves.

The chromatogram produced by the chromatographic analysis will indicatethe concentrations of methyl esters and therefore the relativeconcentrations of their parent fatty acids in the sample. Each esterappears on the chromatogram as a peak at a definite recording time. Thearea under each peak is proportional to the concentration of acid in thesample. The relative concentrations of the even and odd carbon numberfatty acids analyzed may thus be determined from the gas chromatogram.

The above-described analytical procedure should not be considered to bedeterminative with regard to the absolute concentrations of the variousacids in a sample. The procedure may not, and probably in most casesdoes not, recover all of the fatty acid content of the sample. However,it is considered to give extremely accurately results with regard to therelative concentrations of the respective fatty acids in the sample.

Certain factors should be considered in characterizing rocks on thebasis of their fatty acid distributions. In order to avoid possibleerroneous results due to contamination, the ratio between even and oddcarbon number fatty acid concentrations should be determined only withregard to acids having more than 16 carbon atoms and preferably morethan 18 carbon atoms. On the other hand, source rocks contain only smallamounts of fatty acids with 34 or more carbon atoms and their inclusionin determining the acid distribution ratio should have little eifectthereon. Also, the relative concentrations of such acids found in sourcerocks are diflicult to measure with a high degree of accuracy and theirinclusion in determining the acid distribution ratio would more likelylead to error rather than strengthening the results. Thus, it usuallywill be desirable to exclude these high molecular weight acids and it ispreferred in carrying out the present invention to determine the aciddistribution ratio on the basis of relative concentrations of fattyacids having more than 18 carbon atoms and less than 34 carbon atoms.

The acid distribution ratio may be determined for a sample simply bydividing the sum of the relative concentrations of the even carbonnumber fatty acids by the sum of the relative concentrations of the oddcarbon number fatty acids. In order to avoid a distortion of the ratio,it should be determined with regard to the same number of each of thetwo classes of acids. Also, the two classes of acids should be as closeas possible with regard to their molecular weights. For example, if thecumulative relative concentration for even carbon number acids isdetermined on the basis of each even acid within the range of C throughC this value for the odd carbon number acids should be determined on thebasis of each odd acid within the range of C through C or C through C Iffor any reason there is a departure from the above standards for aparticular sample, this same departure should be observed for allsamples correlated therewith in order to achieve comparative uniformity.

The ratio may be determined by other statistically accepted techniques.For example, it may be determined by taking the mean of two ratiosdetermined on the basis of even carbon number acid concentrations over acommon molecular. weight range and odd carbon number acid concentrationsover the two molecular weight ranges most proximate to the common range.The acid distribution ratio (ADR) determined by this technique may bedefined by the following equation:

2 even acid concentrations C' thru CN+M 2 odd acid concentrations C thruC wherein N and M are even integers. For the preferred range givenabove, Equation 1 becomes:

ADR=

y 2 even acid concentrations C2 thru C 2 2 odd acid concentrations Cthru 0 2 even acid concentrations 0 thru C 2 odd acid concentrations Cthru C The acid distribution ratio preferably is determined by thetechnique of Equation 1 since it reduces the effect of unusually largeor small concentrations of odd carbon number fatty acids at the ends ofthe summation ranges. The acid distribution ratio as determined by thistechnique thus provides an indication of the distribution of fatty acidsas it exists over a relatively broad molecular weight range.

A number of various procedures may be followed in practicing the instantinvention. For example, the invention may be utilized as a primaryexploratory technique in order to determine the possibility of petroleumaccumulations within a general geographical area of the earths surface.In this case, it usually will be preferable to obtain samples from alarge number of locations in the earths crust. The samples may beobtained either at the surface of the earth or by drilling to subsurfacelocations in the earths crust. If surface samples are taken, it usuallywill be desired to obtain samples from a depth of at least a few feet inorder to reduce possible anomalous effects introduced by weathering orsurface contamination. Also, the sampling stations may be chosen atrandom or on the basis of a predetermined scheme. For example, insurface surveying, sampling stations may be located at intervals ofabout to 40 feet on surface outcrops of suitable lithologic units.

The samples thus collected are analyzed by the abovedescribed or othersuitable analytical procedure to obtain for each sample the ratiobetween the sum of relative concentrations of a plurality of even carbonnumber fatty acids and the sum of relative concentrations of a pluralityof odd carbon number fatty acids. The ratios for the respective samplesthen are correlated with each other and the locations at the earthssurface at which the samples were taken in order to ascertain possiblesource rocks of petroleum hydrocarbons in the areas. This may beaccomplished by plotting the acid distribution ratio for each sample atits respective location on a geographical map of the area surveyed. Ifthe results of this survey indicate that certain sediment within theearths crusts in the area under investigation may be source rocks,further exploratory measures may then be carried out.

The instant invention also may be utilized as a supplementaryexploratory tool in areas in which other prospecting operations havealready been carried out. For example, seismic surveying of a particularlocality may indicate the presence of subsurface sedimentary structureswithin which petroleum hydrocarbons may be accumulated. In this case,the instant invention can provide a valuable tool in determining whichone of a plurality of surface structures favorable to the accumulationof hydrocarbons may actually contain petroleum deposits.

In this use of the invention, at least one and preferably a plurality ofrock samples may be obtained from each of the rock formations which maybe possible petroleum source rocks. The possible source rock formationsusually will be fine grained sediments such as shales, although in somecases carbonate rocks such as limestones and dolomites may be consideredas possible source rocks. These samples then are analyzed to determinethe ratio between the sum of relative concentrations of even carbonnumber fatty acids and odd carbon number fatty acids. These ratios thenare correlated with each other and the rock formations from which thesamples were obtained to ascertain the rock formation having the lowestacid distribution ratio. This rock formation is considered the mostlikely to have been a source rock from which petroleum hydrocarbons mayhave migrated. The location of this rock formation in the earths crustthen may be correlated with respect to the locations of theaforementioned possible reservoir rocks and a well is drilled into areservoir rock formation which is in fluid communication with thedesignated source rock formation and has a permeability greater than thesource rock formation. Preferably, the well is drilled into a reservoirformation which is in a contiguous relationship with the source rockformation, that is, one that lies next to the source rock and thusprovides a ready acceptor for petroleum formed Within the source rockand migrating therefrom. However, in some cases, it may be desirable todrill a well into a promising rock formation which is not in contiguousrelationship with the source rock, but is in fluid communicationtherewith by other means such as through subterranean faults or joints.

In most cases it will be preferred to designate as source rocks onlythose rock formations having a ratio of even to odd carbon number fattyacid concentrations, preferably as determined in accordance withEquation 1, of about 1.3 or lower, and preferably about 1.2 or lower. Inthis regard, surveys of rock formations identified as source rocks bytechniques independent of the present invention have shown that suchformations almost universally display an acid distribution ratio of notmore than 1.3, and in most cases the acid distribution ratio has been1.2 or less.

Only very rarely has a formation otherwise identified as a source rockbeen found to have an acid distribution ratio of more than 1.3.Notwithstanding this, it may be advisable, under certain circumstances,to consider rock formations having higher such ratios as possible sourcerocks. For example, other exploration techniques such as soil-gasanalysis, etc., carried out in an area may be strongly indicative ofthepropinquity of subsurface petroleum accumulations. However, anexploratory survey in accordance with the instant invention may fail toreveal source-rock type sediments having fatty acid distributions withinthe aforementioned ranges. In this case, in view of the strongindependent evidence of petroleum accumulations, it may be desirable toconsider the rock having the lowest acid distribution ratio as apetroleum source rock even though the ratio of even to odd carbon numberfatty acid concentrations may be greater than 1.3, e.g., on the order of1.6. Thus, in this situation, an exploratory well would be drilled intoa subsurface rock formation which has suitable reservoir characteristicsand is in fluid communication with the designated source rock. Asbefore, the rock formation into which the well is drilled preferablywill be in a contiguous relationshi with the source rock. However, fluidcommunication between the formations may be established by other meanssuch as through faulting.

Having described specific embodiments of the instant invention, it willbe understood that further modifications thereof may be suggested tothose skilled in the art, and it is intended to cover all suchmodifications as fall within the scope of the appended claims.

What is claimed is:

1. In a method of geochemical exploration, the steps comprising:

obtaining rock samples from a plurality of locations in the earthscrust,

analyzing said samples for fatty acids to determine for each of saidsamples the ratio between the sum of relative concentrations of aplurality of even carbon number fatty acids contained therein havingmore than 16 carbon atoms and the sum of relative concentrations of aplurality of odd carbon number fatty acids contained therein having morethan 16 carbon atoms, and

correlating said ratios with each other and the locations in the earthscrust of their respective samples to ascertain source rocks of petroleumhydrocarbons.

2. The method of claim 1 wherein each of said fatty acids has more than18 carbon atoms.

3. The method of claim 1 wherein each of said fatty acids has more than18 carbon atoms and less than 34 carbon atoms.

4. In a method of geochemical exploration, the steps comprising:

obtaining at least one sample from each of a plurality of rockformations in the earths crust,

analyzing said samples for fatty acids to determine for each of saidformations the ratio between the sum of relative concentrations of aplurality of even carbon number fatty acids contained therein and thesum of relative concentrations of a plurality of odd carbon number fattyacids contained therein, correlating said ratios with each other andsaid rock formations to ascertain the one of said rock formations havingthe lowest ratio of even carbon number fatty acid concentrations to oddcarbon number fatty acid concentrations of said plurality of rockformations, and drilling a well into a subsurface reservoir rockformation in fluid communication with said one of said rock formations,said reservoir rock formation having a permeability greater than saidone of said rock formations.

5. The method of claim 4 wherein each of said fatty acids has more than16 carbon atoms.

6. The method of claim 4 wherein each of said fatty acids has more than18 carbon atoms and less than 34 carbon atoms.

7. The method of claim 4 wherein each of said fatty acids has more than18 carbon atoms.

8. The method of claim 7 wherein at least a portion of said reservoirrock formation is in a contiguous relationship with at least a portionof said one of said rock formations.

9. In a method of geochemical exploration, the steps comprising:

obtaining at least one sample from a rock formation in the earths crust,

analyzing said sample to determine the ratio between the sum of relativeconcentrations of a plurality of even carbon number fatty acidscontained therein and the sum of relative concentrations of a pluralityof odd carbon number fatty acids contained therein. and

where the ratio of even carbon number fatty acid concentration to oddcarbon number fatty acid concentration is not greater than 1.3,indicating said formation to be a source rock of petroleum hydrocarbons,drilling a well into a subsurface reservoir rock formation in fluidcommunication with saidsource rock formation, said reservoir rockformation having a permeability greater than said source rock formation.

10. The method of claim 9 wherein said ratio of even carbon number fattyacid concentration to odd carbon number fatty acid concentration is notgreater than 1.2.

11. The method of claim 9 wherein each of said fatty acids has more than16 carbon atoms.

12. The method of claim 9 wherein each of said fatty acids has more than18 carbon atoms and less than 34 carbon atoms.

13. The method of claim 9 wherein each of said fatty acids has more than18 carbon atoms.

14. The method of claim 13 wherein at least a portion of said reservoirrock formation is in a contiguous relationship with at least a portionof said source rock formation.

References Cited UNITED STATES PATENTS 3,305,3 l7 2/1967 Cooper et al.

MORRIS O. WOLK, Primary Examiner E. A. KATZ, Assistant Examiner

